If the sail is downwind of the board and the mast is near the back foot straps, use one arm to hold on to the foot straps (essentially extending the length of the board) and pull the boom over your extended arm to lift the mast and leading edge of the sail from the water. If there is enough wind, fly the sail and do a water start. If the wind is too light to water start, just sit there with the boom resting on your arm until the wind picks up. Half the sail will still be in the water, but in the next gust, it will be easy to fly it out of the water.

Don't lower the boom. If you wear a life jacket, it will make the whole thing a bit easier.

There are other situations with different techniques to clear a sail for a water start.